Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Silica possesses a remarkable variety of polymorphs, summarized in the tablebelow:
|Cristobalite||2.33||Cubic||Above 1470 C|
|Tridymite||2.28||Hexagonal||Above 870 C|
|Quartz (High)||2.53||Hexagonal||Above 570 C|
|Quartz (Low)||2.65||Rhombohedral||Surface Conditions|
|Coesite||2.93||Monoclinic||Above 20 kb|
|Stishovite||4.30||Tetragonal||Above 80 kb|
Cristobalite and tridymite also have high and low forms. The high forms areshown in the diagrams. Low tridymite is orthorhombic and pseudohexagonal, lowcristobalite is tetragonal and pseudo-cubic.
See the discussion of tridymite for an alternative view of the tridymitestability field.
Although the formation temperature of tridymite, 870 C, is well within thenormal magmatic temperature range, tridymite generally forms in silica-richrocks of lower temperature, and must therefore form metastably in most cases.
Many mineralogists, particularly in Europe, follow the lead of Germanmineralogist Otto Floerke and regard tridymite as a polymorph that requiresimpurities (like water) to catalyze its formation, and therefore not a truemember of the phase diagram above. They point to its anomalously low density andthe fact that inversion of pure quartz or cristobalite to tridymite is notobserved in the absence of impurities.
|We can regard tridymite as made of layers of paired tetrahedra, with successive layers alternating in stacking.|
No terrestrial magma gets as hot as the equilibrium formation temperature ofcristobalite (1470 C), and since it is common in cavities in low-temperaturevolcanic rocks like obsidian, as well as some sedimentary settings, it must formmetastably. It probably forms in situations where rapid growth kineticallyfavors a highly open structure, and possibly also by recrystallization ofopaline silica.
Cristobalite also consists of layers of paired tetrahedra, except thesealternate in an ABC cubic pattern.
|Here we see the relationship of the tetrahedra to the cubic unit cell of cristobalite. This is a very open structure. The oxygen layers are alternately 75 and 25 per cent filled compared to cubic close packing.|
Coesite consists of four-membered rings in chains, which in turn arecross-linked by other chains. Two layers are shown below.
In the view below, parts of four layers are shown. This view is slightlyoblique for visibility. In a view directly down the two-fold axis andperpendicular to the glide planes, alternate layers would be directlysuperposed. The green and dark blue layers would be superposed, as would thelight blue and yellow layers.
Stishovite is isostructural with rutile. It forms at pressures so great thatsilica tetrahedra break down and silicon assumes six-fold coordination. In theview below, blue atoms are oxygen, red are silicon.
Silica consists of silicon tetrahedrally coordinated to four oxygen atoms,with silicon atoms linked by intervening oxygen atoms. Ice consists of oxygentetrahedrally coordinated to four hydrogen atoms, with oxygen atoms linked byintervening hydrogen atoms. So it should not be surprising that there is a close- indeed remarkable - similarity between the polymorphs of silica and thepolymorphs of ice. The similarity is purely geometrical, driven by the needto find successively closer packings while maintaining tetrahedral coordination.
Tridymite and cristobalite are related in exactly thesame manner as the low-pressure polymorphs of ice, Ice Ih and Ice Ic, and evenhave identical space groups. Ice II is rhombohedral like quartz, but differs inlacking screw axes in the centers of the six-membered tetrahedral rings.
|Silica polymorph||Space Group||Ice polymorph||Space Group|
|Quartz (low)||R3121||Ice II||R3|
|Tridymite (high)||P63/mmc||Ice Ih||P63/mmc|
|Cristobalite (high)||Fd3m||Ice Ic||Fd3m|
Created 13 March, 2002, Last Update 31 May 2020